1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to a system and method for performing network re-entry due to a handover of a mobile station (MS).
2. Description of the Related Art
Extensive research is being conducted on the next generation communication system to provide users with services that guarantee various Qualities-of-Service (QoSs) at a high data rate.
A wireless Local Area Network (LAN) communication system and a wireless Metropolitan Area Network (MAN) communication system support a high data rate. Compared with the wireless LAN communication system, the wireless MAN communication system, which is a Broadband Wireless Access (BWA) communication system, has broader coverage and supports a higher data rate. Therefore, much of the research on the next generation communication system is being carried out to develop a new communication system that guarantees a mobility and QoS of a subscriber station (SS) for the wireless LAN communication system and the wireless MAN communication system supporting a higher data rate, in order to provide a high-speed service desired by developers of the next generation communication system.
A system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme and/or an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network for physical channels of the wireless MAN communication system is commonly referred to as an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and an IEEE 802.16e communication system, which is based on the IEEE 802.16a and the IEEE 802.16e standards.
FIG. 1 is a diagram illustrating a configuration of a conventional IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a multicell configuration, i.e., includes a cell 100 and a cell 150. The IEEE 802.16e communication system includes a base station (BS) 110 for managing the cell 100, a BS 140 for managing the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. Signal exchange between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 is achieved using the OFDM/OFDMA scheme. Among the MSs 111, 113, 130, 151 and 153, the MS 130 is located in a boundary between the cell 100 and the cell 150, i.e., a handover region. The MS 130 switches its serving BS from the BS 110 to the BS 140 if it moves toward the cell 150 managed by the BS 140 while exchanging signals with the BS 110.
FIG. 2 is a signaling diagram illustrating a network re-entry process with a target BS by an MS due to handover in the conventional IEEE 802.16e communication system.
Referring to FIG. 2, an MS 200, after performing handover from a serving BS to a target BS 250, acquires downlink (DL) synchronization with the target BS 250 and receives in step 211 parameters to be used in a downlink and an uplink (UL). Thereafter, the MS 200 must acquire uplink synchronization and adjust transmission power by performing a ranging operation with the target BS 250. Therefore, the MS 200 transmits in step 213 a Ranging Request (RNG-REQ) message to the target BS 250, and the target BS 250 transmits in step 215 a Ranging Response (RNG-RSP) message to the MS 200 in response to the RNG-REQ message.
After the ranging operation, the MS 200 transmits in step 217 a Subscriber Station Basic Capability Request (SBC-REQ) message to the target BS 250 to negotiate on a basic capability of the MS 200 with the target BS 250. The SBC-REQ message, a Medium Access Control (MAC) message, includes information on a modulation and coding scheme (MCS) supportable by the MS 200. Upon receiving the SBC-REQ message from the MS 200, the target BS 250 transmits in step 219 a Subscriber Station Basic Capability Response (SBC-RSP) message to the MS 200 in response to the SBC-REQ message after checking the MCS that is supportable by the MS 200, included in the received SBC-REQ message.
Upon receiving the SBC-RSP message, the MS 200 transmits in step 221 a Privacy Key Management Request (PKM-REQ) message to the target BS 250 for MS authentication and key exchange. The PKM-REQ message, a MAC message for MS authentication, includes a unique certificate of the MS 200. Upon receiving the PKM-REQ message, the target BS 250 performs authentication on the MS 200 with an Authentication Server (AS, not shown) using the unique certificate of the MS 200, included in the PKM-REQ message. If the MS 200 is an authenticated MS as a result of the authentication, the target BS 250 transmits in step 223 a Privacy Key Management Response (PKM-RSP) message to the MS 200 in response to the PKM-REQ message. The PKM-RSP message includes an authentication key (AK) and a traffic encryption key (TEK) allocated to the MS 200.
Upon receiving the PKM-RSP message, the MS 200 transmits in step 225 a Registration Request (REG-REQ) message to the target BS 250. The REG-REQ message includes MS registration information for the MS 200. Upon receiving the REG-REQ message, the target BS 250 detects MS registration information included in the REG-REQ message, registers the MS 200 therein according to the detection result, and transmits in step 227 a Registration Response (REG-RSP) message to the MS 200 in response to the REG-REQ message. The REG-RSP message includes MS registration information for the registered MS.
After being registered in the target BS 250, the MS 200 can optionally establish in step 229 an Internet Protocol (IP) connection to the target BS 250 depending on a type of the MS 200 or whether information on the MS 200 is shared and exchanged between BSs, or perform in step 231 an operation of transmitting operation parameters to the target BS 250. The operation of establishing an IP connection to the target BS 250 or transmitting operation parameters can be optionally transmitted. Thereafter, the MS 200 re-establishes a connection in step 233 by re-establishing a flow previously being serviced in the serving BS, and in step 235 normally performs a communication service with the target BS 250 through the re-established connection.
FIG. 3 is a signaling diagram illustrating a process of exchanging SBC-REQ message, SBC-RSP message, REG-REQ message and REG-RSP message during a network re-entry process with a target BS by an MS due to handover in a conventional IEEE 802.16e communication system.
Referring to FIG. 3, an MS 300, after performing handover from a serving BS to a target BS 350, transmits in step 311 an RNG-REQ message to the target BS 350 in order to perform a ranging operation with the target BS 350. Then the target BS 350 transmits in step 313 an RNG-RSP message to the MS 300 in response to the RNG-REQ message. Upon receiving the RNG-RSP message, the MS 300 transmits in step 315 an SBC-REQ message to the target BS 350 within a predetermined time. At the time of transmitting the SBC-REQ message to the target BS 350, the MS 300 starts a T18 timer used for waiting for receipt of an SBC-RSP message in response to the SBC-REQ message, waits for receipt of the SBC-RSP message until the T18 timer expires, and retransmits the SBC-REQ message to the target BS 350 if the T18 expires.
However, if the MS 300 receives in step 317 the SBC-RSP message from the target BS 350 before the T18 timer expires, the MS 300 transmits in step 319 an REG-REQ message to the target BS 350 for its registration in the target BS 350. Although an operation of exchanging PKM-REQ/PKM-RSP messages should be performed between the MS 300 and the target BS 350 after the operation of exchanging the SBC-REQ/SBC-RSP messages as described with reference to FIG. 2, the operation of exchanging PKM-REQ/PKM-RSP messages is omitted in FIG. 3 for convenience. At the time of transmitting the REG-REQ message to the target BS 350, the MS 300 starts a T6 timer used for waiting for receipt of an REG-RSP message in response to the REG-REQ message, waits for receipt of the REG-RSP message until the T6 timer expires, and retransmits the REG-REQ message to the target BS 350 if the T6 expires. If the MS 300 receives in step 321 the REG-RSP message from the target BS 350 before the T6 timer expires, it performs the next operation for the network re-entry.
When an MS performs handover from a serving BS to a target BS, it is possible to minimize a service delay due to the handover by minimizing a handover process between the serving BS and the target BS through a backbone network. To this end, the serving BS or the target BS provides the MS with information indicating omittable processes among the processes necessary for the MS and handover of the MS, using a 1-byte Hanover (HO) Process Optimization field. A format of the HO Process Optimization field will now be described with reference to Table 1.
TABLE 1Bit #Description0Omit SBC-REQ/RSP management message during re-entryprocessing1Omit PKM-REQ/RSP management messages during re-entryprocessing2Omit REG-REQ/RSP management message during re-entryprocessing3Omit Network Address Acquisition management messages duringre-entry processing4Omit Time of Day Acquisition management messages duringre-entry processing5Omit TFTP management message during re-entry processing6Full service and operational state transfer or sharing betweenserving BS and target BS (ARQ, timers, counters, MAC statemachines, etc.)7Reserved (if the field is included in NBR-ADV message orMOB-BSHO-RSP message), or Post-HO re-entry MS DL datapending at target BS (if the field is included in RNG-RSP as Type,Length, Value (TLV)
As shown in Table 1, the HO Process Optimization field includes 8 bits, and is used to indicate if the MS should perform various processes while it is performing a network re-entry operation. Each of the 8 bits indicates whether the MS can omit each of the necessary processes while it is performing the network re-entry operation with a target BS after performing handover from a serving BS to the target BS. A description will now be made of information indicated by each of the bits.
First, bit#0 indicates whether to omit transmission/reception of SBC-REQ/SBC-RSP messages between a target BS and an MS. bit#0=‘0’ indicates that transmission/reception of the SBC-REQ/SBC-RSP messages will be performed between the target BS and the MS, and bit#0=‘1’ indicates that transmission/reception of the SBC-REQ/SBC-RSP messages will not be performed between the target BS and the MS.
Second, bit#1 indicates whether to omit transmission/reception of PKM-REQ/PKM-RSP messages between a target BS and an MS. bit#1=‘0’ indicates that transmission/reception of the PKM-REQ/PKM-RSP messages will be performed between the target BS and the MS, and bit#1=‘1’ indicates that transmission/reception of the PKM-REQ/PKM-RSP messages will not be performed between the target BS and the MS.
Third, bit#2 indicates whether to omit transmission/reception of REG-REQ/REG-RSP messages between a target BS and an MS. bit#2=‘0’ indicates that transmission/reception of the REG-REQ/REG-RSP messages will be performed between the target BS and the MS, and bit#2=‘1’ indicates that transmission/reception of the REG-REQ/REG-RSP messages will not be performed between the target BS and the MS.
Fourth, bit#3 indicates whether to omit transmission/reception of Network Address Acquisition management messages between a target BS and an MS. bit#3=‘0’ indicates that transmission/reception of the Network Address Acquisition management messages will be performed between the target BS and the MS, and bit#3=‘1’ indicates that transmission/reception of the Network Address Acquisition management messages will not be performed between the target BS and the MS. The Network Address Acquisition management messages refer to the messages required by the MS to acquire a network address from the target BS.
Fifth, bit#4 indicates whether to omit transmission/reception of Time Of Day Acquisition management messages between a target BS and an MS. bit#4=‘0’ indicates that transmission/reception of the Time Of Day Acquisition management messages will be performed between the target BS and the MS, and bit#4=‘1’ indicates that transmission/reception of the Time Of Day Acquisition management messages will not be performed between the target BS and the MS. The Time Of Day Acquisition management messages refer to the messages required by the MS to newly acquire time information from the target BS.
Sixth, bit#5 indicates whether to omit transmission/reception of Trivial File Transfer Protocol (TFTP) management messages between a target BS and an MS. bit#5=‘0’ indicates that transmission/reception of the TFTP management messages will be performed between the target BS and the MS, and bit#5=‘1’ indicates that transmission/reception of the TFTP management messages will not be performed between the target BS and the MS. Whether to apply the processes related to bit#3, bit#4 and bit#5 to the MS is determined depending on a type of the MS. However, it will be assumed herein that the processes are applied to all MSs regardless of types of the MSs.
Seventh, bit#6 indicates whether an MS can directly perform a normal service in a target BS without any additional process between the MS and the target BS because a serving BS transmits information on the service previously provided to the MS in the serving BS and its operation state information to the target BS or the BSs share the information. bit#6=‘1’ indicates that the MS can directly perform a normal service in the target BS without any additional process between the target BS and the MS. The service and operation state information can include, for example, Automatic Retransmission reQuest (ARQ) state, various timer values, counter values, MAC state machine values, etc.
Finally, bit#7 is reserved (not used) when the HO Process Optimization field is included in a Neighbor Advertisement (NBR-ADV) message and a MOBile Base Station HandOver Response (MOB-BSHO-RSP) message. When the HO Process Optimization field is included in an RNG-RSP message, bit#7 indicates whether a target BS is buffering the downlink data to be transmitted to an MS after the MS performs handover. bit#7=‘1’ indicates that the target BS is buffering the downlink data to be transmitted to the MS after the MS performs the handover.
The HO Process Optimization field, when it is included in the NBR-ADV message and the MOB-BSHO-RSP message as described above, is provided from a serving BS to an MS as a part of information on possible target BSs neighboring the serving BS. When the MS performs handover to a target BS, the meanings indicated by the bits of the HO Process Optimization field may be changed by the target BS. When the HO Process Optimization field is included in the RNG-RSP message, it serves to correctly indicate which process the MS should omit or should not omit during a network re-entry operation to the target BS.
The IEEE 802.16e communication system performs a network re-entry operation with a target BS due to handover of an MS using the HO Process Optimization field information shown in Table 1 as described above, and supports an operation of transmitting one or both of an SBC-RSP message and an REG-RSP message to the MS if needed, regardless of values of bit#0 and bit#2 of the HO Process Optimization field.
However, while the MS is performing network re-entry using the HO Process Optimization field information, if the target BS transmits one or both of an SBC-RSP message and an REG-RSP message to the MS if needed, regardless of set values of bit#0 and bit#2 of the HO Process Optimization field information, the following problems may occur.
First, because the target BS can transmit one or both of an SBC-RSP message and an REG-RSP message to the MS regardless of set values for respective bits of the HO Process Optimization field information included in an RNG-RSP message, even though it is assumed that corresponding bits, i.e., bit#0 and bit#1, of the HO Process Optimization field are set to ‘1’, the MS must wait for receipt of the SBC-RSP message and the REG-RSP message for a predetermined time without performing the next processes for the network re-entry, awaiting the target BS to transmit the SBC-RSP message and the REG-RSP message. In this case, because the MS cannot determine the time for which it must wait for receipt of the SBC-RSP message and the REG-RSP message, it is not possible to satisfy the purpose of the HO Process Optimization field proposed for fast network re-entry with the target BS due to handover of the MS. In addition, the network re-entry operation of the MS has not been clearly specified, causing confusion.
Second, in the state where an MS recognizes that it should not necessarily receive one or both of an SBC-RSP message and an REG-RSP message as one or both of bit#0 and bit#1 of an HO Process Optimization field included in an RNG-RSP message transmitted by a target BS are set to ‘1’ in their values, the MS may occasionally fail to receive one or both of the SBC-RSP message and the REG-RSP message for a possible reason that there is an error in one or both of the SBC-RSP message and the REG-RSP message due to a bad channel state even though the target BS has transmitted one or both of the SBC-RSP message and the REG-RSP message. In this case, the target BS determines that it has successfully transmitted one or both of the SBC-RSP message and the REG-RSP message to the MS, and the MS waits for a predetermined time without performing the next processes for the network re-entry, waiting for the target BS to transmit one or both of the SBC-RSP message and the REG-RSP message, and after a lapse of the predetermined time, performs the next processes for the network re-entry, determining that the target BS has not transmitted one or both of the SBC-RSP message and the REG-RSP message. In this case, the MS fails to receive the values that should be set to different values from the values previously used in a serving BS, from the target BS through one or both of the SBC-RSP message and the REG-RSP message. Therefore, the target BS and the MS are inconsistent in terms of set values of their operation parameters, causing an error in the next processes for the network re-entry. In the worst case, there is a need to initialize a connection between the target BS and the MS. The reason for this problem is because as described for the first problem, the MS cannot determine if the target BS has transmitted one or both of the SBC-RSP message and the REG-RSP message, and even though the MS can determine that the target BS has transmitted one or both of the SBC-RSP message and the REG-RSP message, the MS has no way to determine if the corresponding message(s) was normally received and to report the determination result to the target BS.